Dynamic characterisation of amplifier AM-PM distortion

ABSTRACT

There is disclosed a method of determining an AM-PM distortion measurement for an amplifier, the method comprising: generating a test waveform to be provided to the input of the amplifier; periodically puncturing the test waveform with a fixed-level reference signal to generate a modified test waveform which alternates between test periods in which a portion of the test waveform is present and reference periods in which the fixed-level reference signal is present; measuring the amplifier AM-PM distortion in a test period; measuring the phase difference between the input and the output of the amplifier in reference periods either side of the test period; estimating a phase error in the test period in dependence on phase differences measured in the reference periods; and estimating the true amplifier AM-PM distortion by removing the estimated phase error from the measured amplifier AM-PM distortion.

CROSS-REFERENCE TO RELATED APPLICATIONS

British patent application GB 1205735.2, filed on Mar. 30, 2012, isincorporated herein by reference.

FIELD OF THE INVENTION

The invention relates to the determination of AM-PM distortion of anamplifier.

The invention preferably but not exclusively relates to techniques forachieving efficient amplification of non-constant envelope signals. Theinvention particularly relates to the use of envelope tracking powersupplies for amplification, incorporating shaping functions for shapingthe envelope signal.

The invention preferably, but not exclusively, relates to theamplification of radio frequency (RF) signals.

DESCRIPTION OF THE RELATED ART

Many modern communication systems typically use non-constant envelopemodulation techniques to achieve high spectral efficiency. To avoidspectral spreading into adjacent communication channels, high linearityradio frequency (RF) amplification is required. Traditional fixed biasamplifiers can only achieve the required linearity by ‘backing off’ theamplifier so that it normally operates at a power well below its peakpower capability. Unfortunately, the DC to RF power conversionefficiency in this region is very low. As a consequence these designsdissipate considerable heat and reduce battery life when used inportable applications.

Maximisation of battery life is of paramount importance in mobilewireless equipment for example. With most high spectral efficiencycommunication standards, the mobile transmitter operates at considerablyless than maximum power most of the time. There are two reasons forthis. Firstly, power control is generally used to reduce the averagetransmit power to the minimum level required for reliable communication,and secondly most emerging modulation schemes have a highpeak-to-average power ratio. Hence it is important for the poweramplifier to maintain high efficiency at powers significantly belowmaximum, where the power amplifier operates most of the time.

A known technique for increasing amplifier efficiency, “envelopetracking” (ET), uses a supply modulator to modulate the supply voltagesubstantially in line with the envelope of the input RF signal. Toachieve highest overall efficiency, the efficiency of the supplymodulator itself must be high, requiring the use of a switched modeDC-DC converter within the modulator. The design of the supply modulatoris critical to the system performance of the amplifier. In addition toachieving good efficiency, the modulator must also exhibit highbandwidth, high linearity and low noise to be useful in moderncommunications applications which typically use high bandwidth CDMA orOFDM modulation schemes and also demand high modulation accuracy.

An improved linearization approach uses a non-linear function to derivethe amplifier supply voltage from the input signal envelope to achieveconstant gain from the RF amplifier, thereby reducing the need forpre-distortion or feedback. The mapping function between the envelopevoltage and supply voltage may use a continuous function, in which thesupply voltage may be uniquely derived from knowledge of the envelopevoltage.

In the prior art it is known to characterise a device, to generate ashaping function, by generating a continuous wave signal as an input tothe amplifier. This results in the device getting hot, which can providefalse characterisation data.

In an alternative arrangement, a pulse signal is used to overcome thisproblem associated with a continuous wave signal. However with a pulsewave signal a problem may arise in measuring phase characteristics.

It would thus be advantageous to provide an improved envelope trackingpower amplifier characterisation method to improve the test process.

In known prior art devices, characterisation methods are used inenvelope tracking architectures in order to control the signals in theenvelope path in order to meet a system objective to improve a systemcharacteristic, such as efficiency. This may be at the expense ofaccepting a reduced system performance associated with another systemcharacteristic.

It would thus be advantageous to provide an improved envelope trackingpower amplifier characterisation method and an improved method forutilising the parameters obtained during characterisation to improvesystem performance in use.

In known characterisation methods, obtaining an accurate estimate of theAM-PM distortion of an envelope tracking amplifier is a known problem,as a result of phase wander in the test system itself. Moreover this isa problem which is generally applicable to measuring test parameters foramplifiers in general, irrespective of whether they are part of anenvelope tracking architecture.

It is thus an aim of the invention to provide an improved method fordetermining the AM-PM distortion of an amplifier.

SUMMARY OF THE INVENTION

There is disclosed a method of determining an AM-PM distortionmeasurement for an amplifier, the method comprising: generating a testwaveform to be provided to the input of the amplifier; periodicallypuncturing the test waveform with a fixed-level reference signal togenerate a modified test waveform which alternates between test periodsin which a portion of the test waveform is present and reference periodsin which the fixed-level reference signal is present; measuring theamplifier AM-PM distortion in a test period; measuring the phasedifference between the input and the output of the amplifier inreference periods either side of the test period; estimating a phaseerror in the test period in dependence on phase differences measured inthe reference periods; and estimating the true amplifier AM-PMdistortion by removing the estimated phase error from the measuredamplifier AM-PM distortion.

The step of estimating the phase error may comprise measuring theamplifier phase shift during the reference period either side of thetest period, and using interpolation techniques to estimate the phaseerror during the test period.

The step of measuring the phase difference between the input and theoutput of the amplifier in reference periods either side of the testperiod may comprise, in each reference period either side of the testperiod, measuring a complex signal at the input to and output from theamplifier using measurement receivers, and comparing a captured phase ofthe measured signals in each reference period.

The step of measuring the amplifier AM-PM distortion in a test periodmay comprise measuring a complex signal at the input to and output fromthe amplifier using measurement receivers connected to the input andoutput of the amplifier, and comparing a captured phase of the measuredsignals.

The reference periods either side of the test period may be adjacent tothe test period.

The test waveform may be representative of a signal which is provided asan input to the amplifier in normal use.

BRIEF DESCRIPTION OF THE FIGURES

The invention is now described by way of reference to the accompanyingFigures, in which:

FIG. 1 illustrates an exemplary amplification system in whichimprovements in accordance with the invention and its embodiments may beimplemented;

FIG. 2 illustrates an architecture of an exemplary test/characterisationsystem for characterising a power amplifier device;

FIGS. 3( a), 3(b), and 3(c) illustrate the generation oftest/characterisation waveforms in a preferred arrangement;

FIG. 4 illustrates exemplary shaping functions for atest/characterisation operation;

FIGS. 5( a), 5(b), and 5(c) illustrate exemplary three dimensionalsurfaces generated as a result of the exemplary test/characterisationprocess; and

FIG. 6 illustrates an improved amplification system in accordance with apreferred arrangement.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The invention is now described by way of example with reference toexemplary arrangements. The invention is not limited to the details ofany described arrangement unless expressly stated. Aspects of theexemplary arrangements may be implemented in different combinations, andthe invention is not limited to a specific combination of features byvirtue of the presentation of an exemplary combination for the purposesof explaining the invention.

FIG. 1 illustrates an exemplary envelope tracking radio frequency (RF)power amplifier system 100 in which improvements in accordance with thepresent invention may be implemented. The envelope tracking poweramplifier system 100 includes a power amplifier 102, an up-converter104, an envelope detector 106, a shaping table 108, and an envelopemodulated power supply 110.

A baseband input I/Q signal on line 112 forms an input to theup-converter 104, which generates an RF input signal for the RF poweramplifier on line 122. The instantaneous power of the baseband input I/Qsignal is denoted PIN. The input I/Q signal on line 112 also forms aninput to the envelope detector 106, which generates an envelope signalrepresenting the envelope of the baseband input I/Q signal at its outputon line 116. The envelope detector 106 additionally may generate, asillustrated in the arrangement of FIG. 1, a control signal on line 107to the shaping table 108. In an alternative arrangement this controlsignal for the shaping table may be provided directly from basebandprocessing circuitry (not shown) from which the baseband I/Q signal isderived. The envelope signal on line 116 is provided as an input to theshaping table 108. The output of the shaping table on line 118 providesan input to the envelope modulated power supply, which in dependencethereon provides the supply voltage to the RF power amplifier on line120. The RF power amplifier generates an amplified RF output signal atits output on line 114. The instantaneous power of the RF output signalis denoted P_(OUT).

The up-converter 104 converts the baseband input I/Q signal on line 112into an RF signal for amplification. The envelope detector receives theI/Q signal on line 112, and generates an envelope signal at its outputwhich represents the envelope of the input signal, i.e. provides asignal representing the magnitude of the input signal.

The implementation of the envelope modulated power supply 110 is outsidethe scope of the present invention, and one skilled in the art willappreciate that it may be implemented in a number of ways. Typically theenvelope modulated power supply 110 includes a switched voltage supplyin which one of a plurality of supply voltages can be selected independence on the instantaneous magnitude of the envelope signalprovided by the shaping table. In an efficient amplification scheme, theselected supply voltage may then be further adjusted to provide a moreaccurate representation of the instantaneous envelope signal, beforebeing provided to the RF power amplifier as the supply voltage. Theinvention is not limited to any specific implementation of an envelopemodulated power supply.

The power amplifier 102 may be implemented as a single stage ormulti-stage amplifier.

The shaping table 108 applies a shaping function to the envelope signalon line 116 to provide a shaped envelope signal on line 118. A shapingfunction is a function which maps each instantaneous value of the inputsignal envelope to an instantaneous supply voltage to the amplifier. Theshaping function applied is determined by a control signal received fromthe envelope detector on line 107. The control signal may determinewhich shaping function, amongst a plurality of shaping functions, isapplied to the envelope signal. The control signal may simply be asignal representing the average power of the baseband I/Q input signalon line 112. The shaping of the envelope signal influences theefficiency and linearity of the power amplifier 102.

The power amplifier 102 is characterised in a pre-operation phase todetermine the optimum instantaneous supply voltage level for a givenbaseband input (I/Q) signal level in order to meet particular systemobjectives. This allows an optimum shaping function to be determined andapplied in the shaping table for a given input condition and systemobjective.

A preferred technique for characterising the power amplifier 102 is nowdescribed, initially by describing an exemplary test architecture forthe characterisation process as illustrated in FIG. 2.

The test architecture 200 includes a power amplifier device 102 to becharacterised, and an envelope tracking modulator 110 for the poweramplifier. The envelope tracking modulator 110 may preferably comprise ahigh efficiency envelope tracking modulator, however the invention isnot limited to any specific implementation of an envelope trackingmodulator. The architecture 200 thus includes the set-up of theamplifier and supply which is provided for normal envelope trackingoperation.

The test architecture further includes a digital processor 208. Thedigital processor may comprise part of the digital baseband processingcircuitry provided for normal operation of the amplifier.

The digital processor 208 generates an envelope signal on line 222 tothe envelope modulator 110, and generates a test RF input signal for theamplifier on line 224.

An RF driver 206 amplifies the RF input signal on line 224 for deliveryto the input of the amplifier 102. The RF amplifier amplifies the RFsignal on its input, and generates an RF output signal on line 234.

A plurality of couplers 228, 220, 230 and 260 are provided for detectingthe signal at various points in the test architecture.

The coupler 220 couples to the signal on line 226, and provides on aline 242 an indication of the level of the RF signal at the input to theamplifier. The coupler 228 provides on line 232 an indication of thesignal on line 242. The line 242 is connected to a power meter 218. Theline 232 is connected to the digital processor 208. The digitalprocessor 208 includes a receiver for processing the signal received online 232, and thus includes a receiver for processing the input signalto the power amplifier 102.

The coupler 230 couples to the signal on line 234, and provides on aline 244 an indication of the level of the signal at the output of theamplifier. The coupler 260 provides on line 236 an indication of thesignal on line 244. The line 244 is connected to a power meter 216. Theline 230 is connected to the digital processor 208. The digitalprocessor 208 includes a receiver for processing the signal received online 236, and thus includes a receiver for processing the output signalof the power amplifier 102.

A resistor 210 is connected between the output of the envelope trackingmodulator 110 and a supply input of the power amplifier 102. Adifference amplifier 212 has a pair of inputs connected to therespective terminals of the resistor 210, and an output connected to anoscilloscope 252. A resistor 214 is further connected between the supplyinput to the power amplifier 102 and a further input of the oscilloscope252. The oscilloscope can thus take instantaneous current and voltagemeasurements at the supply terminal of the power amplifier 102.

An RF load 240 is connected to the output of the amplifier 102 on line234, to emulate the load that that would be connected to the output ofthe power amplifier 102 in a practical implementation. A spectrumanalyser 266 may be connected to the load 240 for measuring certainoutput characteristics.

During the characterisation process, the instantaneous RF input power ofthe power amplifier and the instantaneous RF output power of the poweramplifier may be measured using dual ultra linear receivers of thedigital processor 208, which receive representations of the respectiveinput and output power measurements on lines 232 and 236 from therespective couplers 228/220 and 260/230.

Further during the characterisation process, the RF input power of thepower amplifier and the RF output power of the power amplifier may bemeasured using the mean power meters 218 and 216 respectively, whichreceive representations of the respective input and output powermeasurements on lines 242 and 244 from the respective couplers 220 and230.

The power consumption of the power amplifier device may be measuredusing oscilloscope 252, which records both instantaneous current andvoltage.

The digital processor 208 has independent control of the envelope signalto the envelope tracking modulator 110 and the RF input to the poweramplifier 102.

This characterisation of the amplification stage 100 may requiremultiple power sweeps of the amplification stage. This characterisationmay include measurement of various power amplifier parameters, includingsupply voltage; bias voltage; RF gain; RF phase; supply current; RFinput power; and RF output power.

In general, the parameters of the device are measured which arenecessary to determine a particular performance characteristic orobjective. If, for example, it is desired to optimise the gain of theamplification stage, then those parameters necessary to determine gainare measured for different input (envelope) signal and supply voltagecombinations.

A measurement database for a given amplification stage may thus beestablished following a characterisation process. The resultingmeasurement database can be used to predict the operational systemperformance of the device, based on the instantaneous input and thechoice of shaping function.

The characterisation of the amplifier stage is known in the art to becarried out in a number of ways. The present invention presents anadvantageous technique for characterisation which offers improvementsover the prior art.

The architecture of FIG. 2 allows the following characteristic of thepower amplifier device to be measured:

1. Instantaneous output power;

2. Instantaneous input power;

3. Instantaneous output phase;

4. Instantaneous input phase;

5. Instantaneous supply current; and

6. Instantaneous supply voltage.

The instantaneous input and output power measurements are used tocompute AM-AM distortion. The AM-AM distortion of the amplifierrepresents a shift in the relative amplitudes of amplifier input andoutput signals, and may also be referred to as gain distortion. Theinstantaneous input and output phase measurements are used to computeAM-PM distortion. The AM-PM distortion of the amplifier represents ashift in phase delay of a signal between the input and the output of theamplifier, and may also be referred to as a phase distortion. Theinstantaneous supply current and supply voltage are used to computedrain/collector efficiency of the amplifier.

The architecture provides all the “quasi-static” (or memory-less)information for power amplifier device modelling. The architecture alsocaptures the information necessary to generate power amplifier memorymodels.

The detection of average input and output powers using the power meters218 and 216 allows for calibration of the measurement receivers toensure the absolute accuracy of power measurements.

In accordance with an exemplary arrangement, during a test orcharacterisation operation the input signal for the amplifier isprovided by the digital processor as a signal waveform sample which issimilar to a real transmission signal. The actual generation of such asignal is outside the scope of the present invention, and it may begenerated in a number of different ways.

An advantage of using a real transmission signal is that thecharacterisation of the power amplifier occurs under representativeoperating conditions and includes thermal effects. The power amplifieris driven over a range of signals approximating the real operationalconditions, and thermal effects on the power amplifier can therefore betaken into account in the characterisation process. This ensures thethermal loading of the power amplifier during testing/characterisationis representative of that of the power amplifier in normal use.

In an exemplary arrangement, the input waveform used in thecharacterisation process is 10 ms in length.

The test architecture of FIG. 2 permits various characteristics of thepower amplifier 102 to be determined. For example, the instantaneousAM-AM distortion, the instantaneous AM-PM distortion, and theinstantaneous efficiency characteristics may be determined. Thesecharacteristics may be rapidly determined as a function of instantaneousinput power and instantaneous supply voltage.

In the prior art, a difficulty arises in determining AM-PM distortion ofthe power amplifier. Due to ‘phase wander’ in the test architecture overtime, the signals measured at the input and output of the poweramplifier cannot be reliably used to measure the AM-PM distortion of theamplifier, as the phase error caused by phase wander is unknown. Thephase wander refers to a slowly changing (relative to the test signalmodulation) variation in the phase at points in the test architecturedue to low frequency phase noise of the transmitter and receiver localoscillators of the measurement system. For example where separatereceivers are used to receive the input and output signals to the poweramplifier, as illustrated in the exemplary test architecture of FIG. 2,the two receivers may exhibit different phase wander over time, suchthat a phase distortion contribution arises solely from a phase errorbetween the receivers, rather than a AM-PM distortion in the poweramplifier. The amount and effect of the phase wander will depend uponthe test architecture, but would be particularly evident if separatereceivers are used for the input and output of the power amplifier whichare driven from separate local oscillators.

In accordance with a preferred arrangement, an adaptation is made to thecontrol of the test architecture of FIG. 2, as described with referenceto FIG. 3, to enable the AM-PM distortion of the power amplifier to beaccurately determined.

FIG. 3( a) illustrates an exemplary 10 ms test waveform to be providedas an input to the RF driver 206 of FIG. 2 for the purpose ofcharacterising the power amplifier 206. The envelope signal for theenvelope tracking modulator 110 is additionally generated based on thistest waveform. As discussed above, the test waveform of FIG. 3( a)represents a portion of a real signal which would be input to the poweramplifier in use.

In order to allow for an accurate determination of phase distortion, thetest waveform of FIG. 3( a) is punctured using a puncture pattern asshown in FIG. 3( b), to produce a modified test waveform containingperiods of constant voltage as shown in FIG. 3( c).

As can be seen in FIG. 3( b), the puncture pattern divides the timeperiod of the test waveform into a series of sub-periods of equallength. Each alternate sub-period is a puncture period. The otheralternate sub-periods are signal periods, in which the waveform is notpunctured. Although in the described example the sub-periods are ofequal length, in alternative implementations the sub-periods may be ofdifferent lengths.

The resulting waveform as shown in FIG. 3( c) provides sub-periods whichalternate between the presence of the actual waveform signal, which arereferred to as test periods, and periods of constant voltage, which arereferred to as reference periods. In each test period an input signal isprovided which is representative of a real transmission signal asdiscussed above. In each reference period a continuous wave RF signalmay be provided as an input, having a constant amplitude and a constantphase.

In the test periods, when portions of the actual waveform are applied tothe power amplifier, characterisation of the power amplifier 102 takesplace, which preferably includes the acquisition of measurements whichallow the AM-PM distortion of the power amplifier to be determined.

In the reference periods, during which the voltage is constant,information is acquired to determine the portion of the detected AM-PMdistortion which is attributable to phase wander in the testarchitecture. This allows removal of the portion of the phase adjustmentattributable to phase wander from the phase measurements taken in thetest period.

With reference to FIG. 3( c), three successive time periods t₁, t₂, t₃are denoted. Time period t₁ denotes a reference period, time period t₂denotes a following test period, and time period t₃ denotes a followingreference period. In time period t₂ measurements are taken during thetest period which allows the AM-PM distortion of the power amplifier tobe determined. In preceding time period t₁ measurements are also taken,while the supply voltage is constant, to measure the amplifier phaseshift during time period t₁. Similarly in time period time period t₃measurements are also taken, with the same fixed supply voltage, tomeasure the amplifier phase shift in time period t₃. The test periodsare preferably made short enough such that any phase wander over a testperiod can be approximated as a linear function of time. Alternatively,more sophisticated interpolation techniques can be used to estimate thephase error during time period t₂. The difference in measured phasebetween the end of time period t₁ and the start of time period t₃represents the phase wander during time period t₂. The estimated phaseerror during time period t₂ can be removed from the AM-PM distortionmeasurement made in time period t₂, to provide a corrected measurementof AM-PM distortion for time period t₂ which represents an estimate ofthe true AM-PM distortion of the amplifier.

More particularly, linear interpolation or more sophisticatedinterpolation techniques can be used to estimate the phase error in themeasurement system during time period t₂.

Thus an arrangement as described preferably provides a method ofdetermining an AM-PM distortion measurement for an amplifier, the methodcomprising: generating a test waveform to be provided to the input ofthe amplifier; periodically puncturing the test waveform with afixed-level reference signal to generate a modified test waveform whichalternates between test periods in which a portion of the test waveformis present and reference periods in which the fixed-level referencesignal is present; measuring the amplifier AM-PM distortion in a testperiod; measuring the phase difference between the input and the outputof the amplifier in reference periods either side of the test period;estimating a phase error in the test period in dependence on phasedifferences measured in the reference periods; and estimating the trueamplifier AM-PM distortion by removing the estimated phase error fromthe measured amplifier AM-PM distortion.

This method is described herein with reference to an implementationassociated with characterisation of an envelope tracking amplifier, andthe control of an envelope tracking amplifier. However the describedtechnique is more generally applicable, and may be used to determine anAM-PM distortion measurement for any amplifier.

The punctured reference periods in the RF baseband test waveform fordriving the power amplifier under test are preferably used to allow forphase wander compensation, with test periods between these referenceperiods allowing for testing/characterisation of the power amplifier

In practice, a small guard period may be inserted between the referenceand test period to smooth transitions between the two regions. This maybe achieved by smoothing the puncture windows of FIG. 3( b).

In the characterisation of the device, a different predeterminedenvelope tracking shaping function is preferably applied in each testperiod. Each shaping function represents a non-linear transfer function.

An example of such a set of shaping functions is illustrated in FIG. 4.FIG. 4 illustrates a set of shaping functions 40 a to 40 j. As can beseen, each shaping function allows an envelope voltage to be defined foran input signal power level. Thus the envelope voltage applied to theenvelope tracking modulator in dependence on an instantaneous inputsignal is determined by the selected shaping function.

The method of claim 1 or claim 2 wherein the plurality of differentshaping functions comprise a family of functions which increasemonotonically with respect to input signal level.

Each shaping function may be a parameterised algebraic function. Theparameters of each algebraic function may be chosen to characterise theamplifier over a range of combinations of input power and supply voltageof operational interest. The lowest and highest shaping functionsdetermine the range of the characterisation process.

The number of the plurality of shaping functions is preferably chosen tomeet an objective for the resolution of the three dimensional plots. Thelarger the number of shaping functions used in the characterisationprocess, the higher the resolution of the characterisation process.

The shaping function 40 a represents the lowest shaping function of theset, and the shaping function 40 j represents the highest shapingfunction of the set. The set of shaping functions of FIG. 4 are used ina preferred characterisation process. Other shaping functions may beused, and in particular a set of fixed voltages may be used (which wouldbe represented in FIG. 4 by a series of horizontal lines). However thepreferred shaping functions of FIG. 4 provide for a more advantageouscharacterisation process which is representative of normal operationconditions.

The shaping functions of FIG. 4 are algebraic functions with aparameterised swing range. As can be seen each shaping function starts adifferent initial voltage and has a different degree of aggression: inthis context, aggression relates to the variation between the lowestvoltage and highest voltage of the function: the shaping function 40 jis non-aggressive, while the shaping function 40 a is the mostaggressive, applying the lowest possible voltage for a given output.

Thus, in the described example, the instantaneous input signal to thepower amplifier is dependent upon the test waveform as illustrated inFIG. 3( c). The instantaneous supply voltage to the waveform isdependent upon the envelope signal based on the test waveform, aftershaping by the current shaping function applied in the shaping table,according to one of the waveforms of FIG. 4.

In summary, in order to accurately determine the phase distortion of thepower amplifier, the power amplifier under test is driven with awaveform preferably consisting of:

-   -   1. Reference periods which allow phase wander compensation        within the test periods;    -   2. Test periods with different envelope shaping functions        applied, to exercise the power amplifier under test over a large        range of instantaneous supply voltages for a given instantaneous        input power. The test stimuli (i.e. the input waveform) are        chosen to be representative of the statistics of the final        application waveforms of the target system (i.e. to replicate a        real signal); and    -   3. Guard periods, to allow smooth transitions between the        reference and test regions.

It should be noted that this process is required only when it isdesirable to accurately determine the AM-PM distortion. The describedtechniques can still be advantageously used, without puncturing the testwaveform, if such an accurate determination of AM-PM distortion is notrequired. In such case the test waveform is simply divided intosub-periods, a different envelope shaping function is applied line achtest period. The test waveform is continuous, with no reference periodsin such case.

Thus in an arrangement there is provided a method of characterising anenvelope tracking amplification stage comprising an amplifier foramplifying an input signal and an envelope tracking modulated powersupply for generating a modulated supply voltage for the amplifier independence on the input signal envelope, and in which the input signalenvelope to the envelope tracking modulated supply voltage is shaped bya shaping function, the method comprising: generating an input testwaveform which is representative of an input waveform under normaloperating conditions of the amplification stage; applying a respectiveone of a plurality of different shaping functions, each comprising anon-linear transfer function, to the input signal envelope in each of aplurality of test periods during the period in which the input testwaveform is applied as the input signal; measuring parameters of theamplification stage during the period in which the input test waveformis applied in order to allow determination of the gain, phase andefficiency characteristics of the amplifier; and for each of the gain,phase and efficiency characteristics, generating a three dimensionalplot of the characteristic with respect to input power and supplyvoltage applied to the amplifier.

Thus in general, in one arrangement there is provide a technique forestimating the AM-PM distortion of an amplifier, and in anothertechnique there is provided an advantageous technique for characterisingan envelope tracking amplifier using a set of shaping functions. The twotechniques may advantageously be combined, but can be implementedseparately to achieve advantages independently.

During the test or characterisation operation in the preferredarrangement, the following four measurements are preferablysynchronously recorded using the test equipment:

-   -   1. The instantaneous RF input voltage to the power amplifier;    -   2. The instantaneous RF output voltage from the power amplifier;    -   3. The instantaneous supply voltage to the power amplifier (from        an oscilloscope); and    -   4. The instantaneous supply current to the power amplifier (from        an oscilloscope).

Following the capturing of these measurements there may be compiled four‘raw’ waveforms, one for each of the above measurements, during the testor characterisation operation. Thereafter, the four ‘raw’ waveforms areprocessed in a post-processing sequence.

A measurement database for a given amplification stage may thus beestablished following a characterisation process. Such measurementdatabase forms the basis for determining the contents of the shapingtable 108 of FIG. 1. The measurement database can be queried todetermine key aspects of device performance.

In the post-processing operation, the four ‘raw’ waveforms are firstaligned in time and resampled to the same sample rate.

Then phase wander compensation is performed on both the power amplifierinput and power amplifier output receive channels, based on phasemeasurements in the reference periods and compensation during the testperiods as discussed hereinabove.

The raw data of the ‘test periods’ from the four waveforms is thenstripped out.

Next, data fitting to the raw data of the test periods is thenperformed.

Data fitting to the raw data in the test periods may be considered asdirectly analogous to non-memory pre-distortion. The non-memory effectsof the power amplifier may be determined by solving a least squarespolynomial fit to determine polynomial coefficients based on the inputand output data during the test regions. Each test region (with adifferent applied envelope shaping function) is treated independently. Anon-memory model of the power amplifier distortion is used of the form:

${y(n)} = {\left\lbrack \left. {x(n)} \middle| {x(n)} \middle| {}_{0}\mspace{14mu}{x(n)} \middle| {x(n)} \middle| {}_{1}\mspace{14mu}{\ldots\mspace{14mu}{x(n)}} \middle| {x(n)} \right|^{p} \right\rbrack\begin{bmatrix}a_{1} \\a_{2} \\\cdots \\a_{p}\end{bmatrix}}$

where y(n) denotes the current (complex baseband) output from the poweramplifier as detected by a receiver; x(n) denotes the corresponding(time aligned, complex baseband) input sample to the power amplifier asdetected by a receiver; and p is the polynomial order.

Least squares optimisation is performed to determine the coefficients[ao, a1, . . . ap]^(T), given the full set of measurement samples withinthe test period.

Data fitting within the test periods can optionally be extended toinclude memory effects, by adding in memory terms and memorycoefficients into the above least squares data fit expression. This willgive a more accurate model of the power amplifier distortion (includingmemory effects), and hence allow for more accurate predictions of theresponse of the power amplifier.

Alternative data-fitting techniques will be known to one skilled in theart, the above-described technique being a preferred technique.

The data fitting techniques are preferably applied independently to eachtest region in which a different envelope shaping function is applied.

After performing data fitting within the test regions, in accordancewith a preferred implementation smooth three-dimensional surfaces canthen be plotted showing:

-   -   1. Instantaneous gain versus input power and supply voltage;    -   2. Instantaneous phase versus input power and supply voltage;        and    -   3. Instantaneous efficiency versus input power and supply        voltage

Interpolation through any of the three dimensional surfaces then allowsthe gain, phase and efficiency response of the power amplifier to beestimated along any arbitrary envelope shaping function. Alternatively,criteria can be set as a basis for determining an envelope shapingfunction from these measurements, for example maintaining constant gainas a function of instantaneous input power.

FIG. 5( a) illustrates an example surface obtained by this process. Theplot of FIG. 5 (which is a three dimensional surface plot) is a plot ofpower amplifier gain as a function of input power and supplied voltage.As can be seen, the illustrated axes are input power and drain voltage,and the contours of the lines of the plot are contours of the amplifiergain. A shaping function which follows any of the contours will give aconstant gain. The dark line illustrates constant gain as 25 dB.

FIG. 5( b) illustrates a similar plot for power amplifier phase responseas a function of input power and supplied voltage. The dark lineillustrates the phase response associated with constant gain at 25 dB,showing the phase variation following selection of the constant gain inFIG. 5( a).

FIG. 5( c) illustrates a similar plot for power amplifier efficiency asa function of input power and supplied voltage. The dark line shows theefficiency response associated with constant gain at 25 dB, showing thephase variation following selection of the constant gain in FIG. 5( a).This allows an assessment to be made as to how close operation is tomaximum efficiency.

It can be seen that the characterisation of the power amplifier thusallows three dimensional surfaces to be created which show the responseof the power amplifier to any arbitrary envelope shaping function for agiven system characteristic (such as gain, phase or efficiency). In thepreferred arrangement the phase, gain and efficiency responses aremeasured to provide three three-dimensional surfaces. Eachthree-dimensional surface comprises a plot for the given systemcharacteristic (gain, phase, efficiency) against input power and supplyvoltage to the amplifier.

In accordance with a preferred arrangement, the shaping function can bechosen to satisfy some particular criteria, such as maximum efficiencyfrom the power amplifier. The gain and/or phase surfaces may then beused to determine the residual gain and/or phase error resulting fromthat chosen shaping function to maximise efficiency. This can beunderstood from FIGS. 5( a) to 5(c) above, where FIG. 5( a) shows theselection of a shaping function to meet a constant gain response, andFIGS. 5( b) and 5(c) show how applying that shaping function in theshaping table will impact on the phase and efficiency response of thepower amplifier.

Based on knowledge of the residual errors once a system objective isachieved with selection of the shaping function, in accordance with apreferred arrangement a pre-distortion block in the RF signal input pathcan be adjusted to compensate for PM and/or AM residual errors over someor all of the power range of the RF signal.

FIG. 6 illustrates a modification to the amplifier arrangement of FIG. 1to allow for this preferred arrangement. As can be seen in FIG. 6, apre-distortion block is provided in the RF input path, to receive theinput I/Q signals on line 112 and to generate an output signal on line123 which provides an input to the up-conversion block 104. The envelopedetector 106 generates an additional control signal 125 which controlsthe pre-distortion block 121. The control signal on line 125 may berepresentative of the average power of the input signal on line 112. Inan alternative arrangement, this control signal on line 125 for thepre-distortion block may be provided directly from baseband processingcircuitry (not shown).

Thus in a general arrangement there is provided a method of controllingan envelope tracking amplification stage comprising an amplifier foramplifying an input signal, an envelope tracking modulated power supplyfor generating a modulated supply voltage for the amplifier independence on the input signal envelope, and in which the input signalenvelope to the envelope tracking modulated supply voltage is shaped bya shaping function, and a pre-distortion block for pre-distorting theinput signal to the amplifier, the method comprising: in acharacterisation mode of the amplifier under test conditions: measuringparameters of the amplification stage to determine at least two of gain,phase and efficiency characteristics for instantaneous values of inputpower and supply voltage of the amplifier; and for each of the at leasttwo of gain, phase and efficiency characteristics, generating athree-dimensional plot representing the characteristic with respect toinput power and supply voltage applied to the amplifier, and in a usemode of the amplifier under normal operating conditions: using at leastone of the three-dimensional plots to determine a shaping function forthe shaping table in dependence on a primary system objective associatedwith one or more of gain, phase or efficiency; and using the determinedshaping function and at least one of the three dimensional plots todetermine the pre-distortion coefficients for the pre-distortion blockto meet a secondary system objective associated with at least one ofgain, phase or efficiency.

In the arrangement of FIG. 6, the shaping function in the shaping table108 can be selected from one or more of the phase, gain and efficiencyplots of FIGS. 5( a) to 5(c) in order to meet a primary system objectiveassociated with one of those plots. For example, the shaping functionmay be selected from the efficiency plot, in order to meet an efficiencyobjective. Once the shaping function is selected, then pre-distortioncoefficients for the pre-distortion block 121 can be determined from thephase and gain plots in order to meet a secondary system objective.Where the primary system objective is an efficiency objective, thesecondary system objective may be a gain and/or phase objective.

As noted above, when the shaping function is selected to achieve anefficiency objective, for example, the effect of this on the gain and/orphase performance can be determined by tracking that shaping function inthe phase and/or gain three-dimensional surface plots. This can then beused to determine the difference or error between the actual phase orgain response associated with meeting the first system objective, andthe desired phase or gain response associated with the second systemobjective. The resulting difference or error arising with respect to thesecond system objective can then be cancelled, or at least mitigated tosome extent, by making an appropriate adjustment in the coefficients ofthe pre-distortion block 121. The residual gain and/or phase errors (ordifferences in comparison to a secondary system objective) can forminputs to look-up tables of the pre-distortion block.

The contents of these look-up-tables may be switched based on the chosenenvelope shaping table (since the residual gain and phase errors willalso change based on chosen envelope shaping functions).

This may allow, for example, for a constant phase to be achieved byapplying in the pre-distortion block a phase function which is inverseto the phase function resulting from the selection of the shapingfunction. For example, a secondary system objective may be constantgain, but in fulfilling a first system objective associated withefficiency it can be determined from the three-dimensional gain surfacethat as a result gain will not be constant. A function which inverts theeffect on gain of meeting the primary system objective may be applied inthe pre-distortion block if the amplifier is operated in mildcompression.

It should be understood, however, that the above example of a secondarysystem target of a constant gain is only one example. Any gain objective(or any other objective such as a phase objective) may be required as asecondary system response. An appropriate adjustment can preferably bemade in the pre-distortion block to compensate as desired according toan implementation. For example, alternatively it may be an objective tocorrect the phase over the entire RF signal range, but to only correctthe amplitude in the low power range (i.e the range of operation wherethe amplifier is not in saturation).

By way of further example, reference is made to a further modification.The shaping function may be determined to meet a primary systemobjective, as described above (for example an efficiency objective), insuch a way as to reduce the crest factor of the RF output signal as ameans of achieving a further improvement in amplifier efficiency. Crestfactor reduction is an example of such a modification, but in otherimplementations some other adaptation to the shaping function selectedto meet the desired primary system objective may be required. In suchcase, the pre-distortion block 106 is not intended to remedy or inversethe effect resulting from the deliberate modification to the shapingfunction. In the example given, a controlled amount of AM distortion athigh signal powers is deliberately introduced in order to reduce theCrest Factor of the RF output, and the pre-distortion block should notremove this AM distortion. The pre-distortion block will be arranged tomake an appropriate adaptation to meet a secondary system objective,whilst maintaining the effect of any additional modificationintentionally made to the selected shaping function.

The pre-distortion block 106 can only be used to correct for AM-AMdistortion (i.e. gain distortion) at low power, i.e. when the amplifieris not heavily compressed. The pre-distortion block can be used tocorrect phase over all power ranges, i.e. whether the amplifier is incompression or not. In general, it would generally be desirable tocorrect phase over the whole power range, whereas gain would only becorrected when the amplifier is not in compression.

Some specific examples associated with the secondary objective are nowdefined. The secondary objective may be a linearity objective tominimise the AM-AM and AM-PM distortion of the amplifier. The secondaryobjective may be a linearity objective to minimise at least one of theAM-AM and AM-PM distortion of the amplifier over at least part of therange of input values. The secondary objective may be a linearityobjective to linearise AM-PM distortion of the amplifier over the entirerange of input values. The secondary objective may be a linearityobjective to linearise AM-PM distortion of the amplifier over the entirerange of input values, and to linearise AM-PM distortion at low inputpower levels.

In the foregoing it is discussed that the pre-distortion coefficients inthe pre-distortion block are adjusted. In a practical implementation,the pre-distortion block may be a look-up-table (LUT) based entity, withat least one complex LUT (or gain and phase) as a function of inputpower, i.e. directly analogous to the gain and phase surfaces asdescribed above.

The LUT is preferably derived from coefficients that are extracted fromthe non-memory polynomial fit during surface extraction as describedabove. Thus reference to adjusting the coefficients in thepre-distortion block can be understood as incorporating adjusting thevalues in the LUTs in the pre-distortion block.

Thus adjusting the pre-distortion blocks in the pre-distortion block maycomprise adjusting the pre-distortion LUTs in the pre-distortion block,in an exemplary implementation. The use of the envelope trackingarchitecture incorporating the power amplifier and envelope trackingmodulator, in combination with the baseband digital processor, allowsthe mode of the architecture to be switched between a characterisationor test mode and an operational mode.

The preferred arrangements in accordance with the invention haveparticular applicability, and offer particular improvements, in handsetapplications in comparison to prior art arrangements, since it is a fastcharacterisation technique which is representative of the condition thehandset will operate in.

The invention and its embodiments and arrangements has applicability ingeneral for any amplification stage incorporating an envelope trackingmodulated power supply. Example implementations include mobilecommunications systems, both in wireless handsets and in wirelessinfrastructure.

The methods described herein may be embodied in computer software, suchas a computer program comprising computer program code, which whenexecuted on a computer causes a processor or processing elementsassociated with the computer to operate in accordance with the describedmethods. Such a computer program may be stored on a computer programproduct, such as a disc or other memory device.

Whilst the invention has been described with relation to particulararrangements, the invention is not limited to any specific combinationof features or any specific feature unless defined as such by theappended claims. One skilled in the art will appreciate variations andmodifications in the described arrangements which will fall within thescope of the defined invention.

What is claimed is:
 1. A method of determining an AM-PM distortionmeasurement for an amplifier, the method comprising: generating a testwaveform to be provided to the input of the amplifier; periodicallypuncturing the test waveform with a fixed-level reference signal togenerate a modified test waveform which alternates between test periodsin which a portion of the test waveform is present and reference periodsin which the fixed-level reference signal is present; measuring theamplifier AM-PM distortion in a test period; measuring the phasedifference between the input and the output of the amplifier inreference periods either side of the test period; estimating a phaseerror in the test period in dependence on phase differences measured inthe reference periods; and estimating the true amplifier AM-PMdistortion by removing the estimated phase error from the measuredamplifier AM-PM distortion.
 2. The method of claim 1 wherein the step ofestimating the phase error comprises measuring the amplifier phase shiftduring the reference period either side of the test period, and usinginterpolation techniques to estimate the phase error during the testperiod.
 3. The method of claim 1 wherein the step of measuring the phasedifference between the input and the output of the amplifier inreference periods either side of the test period comprises, in eachreference period either side of the test period, measuring a complexsignal at the input to and output from the amplifier using measurementreceivers, and comparing a captured phase of the measured signals ineach reference period.
 4. The method of claim 1 wherein the step ofmeasuring the amplifier AM-PM distortion in a test period comprisesmeasuring a complex signal at the input to and output from the amplifierusing measurement receivers connected to the input and output of theamplifier, and comparing a captured phase of the measured signals. 5.The method of claim 1 wherein the reference periods either side of thetest period are adjacent to the test period.
 6. The method of claim 1wherein the test waveform is representative of a signal which isprovided as an input to the amplifier in normal use.
 7. A non-transitorycomputer program including non-transitory computer program code storedon a non-transitory computer program product which, when run on acomputer system, performs the method of claim
 1. 8. A non-transitorycomputer program product for storing non-transitory computer programcode which, when run on a computer system, performs the method ofclaim
 1. 9. A measurement system for determining an AM-PM distortionmeasurement for an amplifier, the measurement system being adapted to:generate a test waveform to be provided to the input of the amplifier;periodically puncture the test waveform with a fixed-level referencesignal to generate a modified test waveform which alternates betweentest periods in which a portion of the test waveform is present andreference periods in which the fixed-level reference signal is present;measure the amplifier AM-PM distortion in a test period; measure thephase difference between the input and the output of the amplifier inreference periods either side of the test period; estimate a phase errorin the test period in dependence on phase differences measured in thereference periods; and estimate the true amplifier AM-PM distortion byremoving the estimated phase error from the measured amplifier AM-PMdistortion.
 10. The measurement system of claim 9 further adapted toestimate the phase error by measuring the amplifier phase shift duringthe reference period either side of the test period, and useinterpolation techniques to estimate the phase error during the testperiod.
 11. The measurement system of claim 9 further adapted to measurethe phase difference between the input and the output of the amplifierin reference periods either side of the test period by, in eachreference period either side of the test period, measuring a complexsignal at the input to and output from the amplifier using measurementreceivers.
 12. The measurement system of claim 9 further adapted tomeasure the amplifier AM-PM distortion in a test period by measuring acomplex signal at the input to and output from the amplifier usingmeasurement receivers connected to the input and output of theamplifier, and comparing a captured phase of the measured signals. 13.The measurement system of claim 9 wherein the test waveform is arepresentation of a signal which is provided as an input to theamplifier in normal use.
 14. The measurement system of claim 9 whereinthe reference periods either side of the test period are adjacent thetest period.
 15. A measurement system for determining an AM-PMdistortion measurement for an amplifier, the measurement systemcomprising: means for generating a test waveform to be provided to theinput of the amplifier; means for periodically puncturing the testwaveform with a fixed-level reference signal to generate a modified testwaveform which alternates between test periods in which a portion of thetest waveform is present and reference periods in which the fixed-levelreference signal is present; means for measuring the amplifier AM-PMdistortion in a test period; means for measuring the phase differencebetween the input and the output of the amplifier in reference periodseither side of the test period; means for estimating a phase error inthe test period in dependence on phase differences measured in thereference periods; and means for estimating the true amplifier AM-PMdistortion by removing the estimated phase error from the measuredamplifier AM-PM distortion.